MONTREAL -- A battle for control of Quebec's biggest cable-TV company is highlighting the outsize role of a public pension-fund agency in the province's economy and reviving controversy over what critics call its provincial behavior.

In a case that may offer a preview of the possible role of social security funds in stock markets elsewhere, the pension agency, the Caisse de Depot & Placement du Quebec, is trying to thwart the acquisition of Montreal cable company Groupe Videotron Ltd. by Toronto's Rogers Communications...